I. Field of the Invention
The present invention pertains to a method for making high density nickel hydroxide for alkali rechargeable batteries. More particularly, the present invention pertains to a method for making nickel hydroxide for a battery electrode from a secondary low cost nickel source.
II. Description of the Background Art
The demand for batteries has grown dramatically over the past decade and continues to grow at a phenomenal rate. Rechargeable batteries with high energy density and high capacity are particularly desirable. Two types of batteries that are widely used are the Nixe2x80x94Cd (nickel cadmium) type and the more desirable Ni-MH (nickel metal hydride) type. These batteries have a positive and negative electrode. In both types of batteries the positive electrodes are made primarily of nickel hydroxide active material.
Ni-MH cells utilize a negative electrode that is capable of the reversible electrochemical storage of hydrogen. Ni-MH cells usually employ a positive electrode of nickel hydroxide material. The negative and positive electrodes are spaced apart in an alkaline electrolyte. Upon application of an electrical potential across a Ni-MH cell, the Ni-MH material of the negative electrode is charged by the electrochemical absorption of hydrogen and the electrochemical discharge of a hydroxyl ion, as shown in equation 1.
M+H2O+exe2x88x92⇄M-H+OHxe2x88x92xe2x80x83xe2x80x83(1):
The negative electrode reactions are reversible. Upon discharge, the stored hydrogen is released to form a water molecule and release an electron.
The reactions that take place at the nickel hydroxide positive electrode of a Ni-MH cell are shown in equation 2.
Ni(OH)2+OHxe2x88x92⇄NiOOH+H2O+exe2x88x92xe2x80x83xe2x80x83(2):
The use of nickel hydroxide, Ni(OH)2, as a positive electrode material for batteries is generally known. See for example, U.S. Pat. No. 5,523,182, issued Jun. 4, 1996 to Ovshinsky et al., entitled xe2x80x9cEnhanced Nickel Hydroxide Positive Electrode Materials For Alkaline Rechargeable Electrochemical Cellsxe2x80x9d, the disclosure which is herein incorporated by reference.
Several forms of positive electrodes exist at the present and include sintered, foamed, and pasted electrode types. Processes for making positive electrodes are generally known in the art, see for example U.S. Pat. No. 5,344,728 issued to Ovshinsky et al., the disclosure of which is herein incorporated by reference, where capacity in excess of 560 mAh/cc was reported. The particular process used can have a significant impact on an electrode""s performance. For example, conventional sintered electrodes normally have an energy density of around 480-500 mAh/cc. Sintered positive electrodes are constructed by applying nickel powder slurry to a nickel-plated, steel base followed by sintering at high temperature. This process causes the individual particles of nickel to weld at their points of contact, resulting in a porous material that is approximately 80% open volume and 20% solid metal. This sintered material is then impregnated with active material by soaking it in an acidic solution of nickel nitrate, followed by the conversion to nickel hydroxide by reaction with an alkali metal hydroxide. After impregnation, the material is subjected to electrochemical formation.
To achieve significantly higher loading, the current trend has been away from sintered positive electrodes and toward pasted electrodes. Pasted electrodes consist of nickel hydroxide particles in contact with a conductive network or substrate, most commonly foam nickel. Several variants of these electrodes exist and include plastic-bonded nickel electrodes, which utilize graphite as a microconductor, and pasted nickel fiber electrodes, which utilize spherical nickel hydroxide particles loaded onto a high porosity, conductive nickel fiber or nickel foam support.
The production of low cost, high capacity nickel hydroxide is critical to the future commercialization of Ni-MH batteries. As with electrode formation, the properties of nickel hydroxide also differ widely depending upon the production method used. Generally, nickel hydroxide is produced using a precipitation method in which a nickel salt and a hydroxide salt are mixed together followed by the precipitation of nickel hydroxide. Active, nickel hydroxide material preferably has high capacity and long cycle life, see U.S. Pat. No. 5,348,822 to Ovshinsky et al., the disclosure of which is herein incorporated by reference.
It has been discovered that nickel hydroxide suitable for use in a battery electrode should have an apparent density of 1.4-1.7 g/cm3, a tap density of about 1.8-2.3 g/cm3, and a size range of about 5-50xcexc. Active, nickel hydroxide particles are preferably spherical in shape with a high packing density and a narrow size distribution Preferably, average particle size should be about 10 xcexcm and tap density should be about 2.2 g/cc. Paste made with this kind of nickel hydroxide has good fluidity and uniformity, and thus it is possible to fabricate high capacity, uniformly loaded electrodes. The use of this kind of nickel hydroxide also improves the utilization of the active material and discharge capacity of the electrode. If the process is not carefully controlled, the precipitate will have an irregular shape and/or low tap density. For example, if the rate of reaction is too fast, the precipitate formed may be too fine and the density too low. A fine powder with low density requires longer filtering or washing times and increases the adsorption of water on the surface. Further, if the precipitated particles have too wide a size distribution (ranging from 1 to hundreds of microns), the nickel hydroxide may require pulverization to render it useful. Electrodes formed with low-density nickel hydroxide will lack high capacity and high energy density. For these reasons and others, an active powder having an irregular shape and/or low density is less than desirable for use as a high capacity battery electrode material.
In order to produce high density, substantially spherical nickel hydroxide, particles are gradually grown under carefully controlled process conditions. A nickel salt provided in solution is combined with an ammonium ion. The nickel salt forms complex ions with ammonia to which caustic is added. Nickel hydroxide is then gradually precipitated by decomposition of the nickel ammonium complex. The reaction rate is difficult to control, so methods have been introduced to separate critical steps in the production process to compensate for said difficulties. For example, U.S. Pat. No. 5,498,403, entitled xe2x80x9cMethod for Preparing High Density Nickel Hydroxide Used for Alkali Rechargeable Batteriesxe2x80x9d, issued to Shin on Mar. 12, 1996, the disclosure of which is herein incorporated by reference, discloses a method of preparing nickel hydroxide from a nickel sulfate solution using a separate or isolated amine reactor. Nickel sulfate is mixed with ammonium hydroxide in the isolated amine reactor to form a nickel ammonium complex. The nickel ammonium complex is removed from the reactor and sent to a second mixing vessel or reactor where it is combined with a solution of sodium hydroxide to obtain nickel hydroxide. Such a method relies heavily on a raw material source of very high purity or what is termed throughout the ensuing specification as primary nickel.
Thus, particular notice should be taken in the fact that all of present day processes for making positive electrode materials, such as those described above, have utilized expensive, high grade, and highly pure primary nickel for the production of nickel salt starter solutions. As modem process technology and automation have reduced the cost of labor in the production of battery electrode materials, the cost of primary nickel and its associated salts have become a significant factor in determining the cost of active electrode materials, battery electrodes, and the batteries the electrodes are placed within, making up as much as 60% of the direct manufacturing cost of the final nickel hydroxide.
Primary nickel used for the production of active materials is typically derived from the ores of nickel sulfide and nickel oxide and purified by electro-processes. Nickel sulfide ores are refined by flotation and roasting to nickel oxide. Nickel oxide ores are typically refined by hydrometallurgical refining, such as leaching with ammonia. Refined nickel ore is usually cast into nickel anodes for distribution as primary nickel. The highly pure, primary nickel may then be dissolved into solution, such as a sulfate solution, and sold as highly pure aqueous nickel sulfate, with a frequent end use also being nickel electroplating and electroless nickel plating.
The average amount of nickel estimated to be present in the earth""s crust is only about 0.0084 wt %, as reported on page 14-14 of the Handbook of Chemistry and Physics, 78th Edition, 1997-1998. Because nickel is used for many things, including the production of stainless steel, the demand for nickel is very high, making it a relatively expensive metal. Although primary nickel is a commodity product, it is subject to wild market swings in price. For example, during the period of Jun. 1, 1999 through Jun. 1, 2000, nickel prices have seen dramatic volatility having a low of 2.16 $/lb and a high of 4.77 $/lb as reported on the London Metal Exchange. As a means of off-setting or hedging against the increasing cost of nickel, a number of large producers of nickel hydroxide have gone so far as to purchase ownership interests in nickel mines. Smaller manufactures of nickel hydroxide, unable to offset rising nickel prices, have been left at a competitive disadvantage.
Thus, present day methods of producing nickel hydroxide from highly pure nickel lack a material independent source of nickel that is not driven by the market costs of primary nickel.
One particular source of nickel not presently utilized for the production of nickel hydroxide for battery electrodes is that of secondary nickel or nickel by-product. Secondary nickel is that nickel which is derived from either process or waste streams unrelated to primary nickel or the production of high purity nickel, or is nickel from spent or virgin solutions used in electroplating or electroless plating of nickel. One way to characterize secondary nickel is by its history of use. Although present methods may exist for refining nickel, see for example U.S. Pat. No. 5,861,131, the disclosure of which is herein incorporated by reference, such methods do not provide a secondary nickel source of suitable quality for the production of nickel hydroxide materials used in battery electrode materials. Additionally, the background art fails to teach or suggest the use of any secondary nickel or nickel by-products for use in active battery electrode materials, especially nickel hydroxide production. While these secondary nickel sources cannot be classified entirely as waste, the cost of using secondary nickel dramatically reduces overall cost of active materials.
Thus, there exists a long felt and presently unfulfilled need for an alternative to primary nickel for the production of battery electrodes and electrode materials.
The subject invention addresses the above stated problems and others by, among other things, providing a method for making nickel hydroxide using a novel starting material. Nickel electroplating and electroless plating solutions, including both virgin and spent solutions and various types of waste streams having metal ions are generally known but have not heretofore been used to produce active, battery electrode materials. Thus, the present invention provides a new use for a secondary metal or metal waste stream, by using the metal as a starter material for the production of an active, positive electrode material.
The battery electrode material is preferably a substantially spherical, high density nickel hydroxide material or nickel oxyhydroxide material, which may comprise one or more modifiers or modifier elements. Preferred modifier elements include those selected from the group consisting of Al, Ba, Bi, Ca, Co, Cr, Cu, F, Fe, In, K, La, Li, Mg, Mn, Na, Ru, Sb, Sn, Sr, Ti, and Zn, etc. In particular, modifiers of Co, Zn, Ca, Mg, Cu, Al and Li are more preferred.
Generally, the invention provides a method for making nickel hydroxide battery electrode materials from secondary nickel or a secondary nickel source where nickel is a by-product.
The method includes the steps of: providing secondary nickel as a source of nickel; formulating the secondary nickel into a solution; adding a precipitating agent to the solution in an amount effective to precipitate a nickel salt; separating the nickel salt from the solution; dissolving the separated nickel salt into a nickel salt solution; evaporating a portion of the nickel salt solution to precipitate a nickel salt precipitate; and converting the nickel salt precipitate into an active positive electrode material.
A second aspect of the invention recognizes that to utilize the secondary nickel source, the precipitation process is crucial to the ultimate suitability of the nickel hydroxide end product. Thus, a single precipitation reaction, instead of the common two reactor system with a preamine initial reaction, allows the use of a secondary nickel source.
A third aspect of the invention recognizes that the utilization of a secondary nickel source need not be an all or nothing proposition, and that particular end formulas of the nickel hydroxide may have more or less tolerance to the secondary nickel source. The method thereby recognizes that the xe2x80x9cas isxe2x80x9d secondary nickel source may be substituted at least in part due to the presence of impurities and concentration difficulties. A novel approach of blending solutions using primary and secondary nickel is disclosed.
A fourth aspect of the invention recognizes that the battery end use may dictate certain performance properties, such as paste loading and capacity. In this case, a novel approach of blending nickel hydroxide made from primary nickel together with nickel hydroxide made from secondary nickel is disclosed.
Still, another aspect of the invention is to recycle xe2x80x9cout of specification nickel hydroxidexe2x80x9d back into nickel sulfate solution suitable for making new nickel hydroxide. The xe2x80x9cout of specification nickel hydroxidexe2x80x9d may be reactor startup scrap, transition material formed between chemical formula changes, or material resulting from production issues such as power outages, equipment failure, etc.
Nickel hydroxide material produced in accordance with the present method provides particles having a shape, a particle size, a tap density, and a crystallinity suitable for use as an active positive electrode material. For a more complete understanding of the present invention, reference is made to the following detailed description and accompanying drawings.